Process of purifying gas materal



Juy24,1934. L BOWDIT H I 1,967,736

PROCESS OF PURIFYING GAs MATERIAL Filed April 25, 19352. 2 Sheets-Sheet l L. IQLJID- p F E /A m z B'/'// A 5 E, SOLID GAS TEMPEIRATURE T1 T2 INVENTOR Q B MQMM m 1934s L. J. BOWDITCH 1,967,736

PROCESS OF PURIEYING GAS MATERIAL Filed April 25, 1932 2 Sheets-Sheet 2 INVENTOR BY X v I f Ag r 'zRNEYs I such as liquid air, liquid oxygen and liquid carpatented 24, 134

PRQCESS (13F PURKFYHNG GAS IWA'ITIERML Lamence J5. Flushing, N.

by mesne assignments, to The Linda Air Fred-=- nro filempany, New York, 1., a corporation 3 Claims. (G31. 18%115) This invention relates to a process of purifying gas material and particularly gas material which has been collected as a by-product and contains one or more undesired impurities, and has for its object generally an improved procedure whereby such impurities are readily and expeditiously removed.

More specifically, the invention relates to the removal of impurities which occur in relatively small amounts in liquefied gases, such as liquefied air,'oxygen, carbon dioxide, and the like,

their concentrations being such as a rule thatv they do not materially change the boiling point of the liquefied gas, but are otherwise objectionable and impair the commercial qualities of the gas product.

A specific object is to accomplish the substantially complete removal of these impurities with a relatively small expenditure of energy and at relatively little cost.

connection with the accompanying drawings, in-

which:

Fig. 1 is aview mainly in vertical section showing a simple form of apparatus adapted for carrying out the process of the present invention;

Fig. 2 is an explanatory diagram; and V Fig. 3 is a view mainly in vertical section showing apparatusfor removing more than one variety of impurity, in accordance with the present invention.

In the commercial production of liquefied gases, undesirable impurities are generally present, both by reason of the sources from which the gas is obtained and on account of the manufacturing steps commonly employed to produce initially the liquid state or phase. Such impurities, though present even in so small an amount as not to change appreciably the boiling point of the liquefied gas, are detectable in the final product, and are commercially objectionable. This is particuiarly true in the production of liquefied gases,

bon dioxide. 3

"these impurities have heretofore been removed mercial method for continuously removing all such impurities. The renewal of these agents also imvolves a considerable item of expense.

Certain types of undesired impurities so incorporated with the gas material in commercial practice are less volatile than the gas material, that is, having melting points materially above the boiling point of the gas material. To accomplish the removal of these less volatile impurities, in accordance with the present invention, gas material which contains the undesired 1mpurities is brought to a condition by the step of fractional evaporation, under conditions of temperature and pressure where the vapor pressure of the impurity is of negligible value (i. e., a. value, which when expressed in customary units is of an order practically incapable of measurement in commercial practice) such that separation may be produced physically, the impurities remaining 7 in the liquid or solid phase. The purified gas fraction is then separated therefrom in any convenient state, for example in the gas phase.

Where the gas material initially contains various impurities, some of which are more volatile and others less volatile than the gas material, the purifying process of the present invention may be practiced in two stages, in the first of which the more volatile impurities are drawn on in the gas phase, so that the liquid fraction remaining comprises not only the desired gas material, but also the less volatile impurities. In the second stage, fractionation is practiced as before, whereby the purified. gas material may be withdrawn in the gas phase.

In order that this'purifying process may be practiced with the expenditure of relatively little energy, the gas material which contains the undesired impurities is passed into a heat exchanger and brought substantially to a condition where the less volatile impurities tend to separate out intheir liquid or solid phases. Hence, the fractionation desired may be obtained merely by theexchange of a certain amount of latent heat to produce the phase change involved.

Referring now to the drawings and particularly to Fig. 1, 10 denotes generally a heat exchanger having a chamber 11 supplied with the gas material' through the conduit 12. A heat exchanging jacket about the chamber 11 is shown at 13, provided with inlet and outlet connections 14 and 15, respectively. The chamber 11 has an outlet conion duit 16 leading to the phase separator shown generally at 17.

The flow of gas materiaLcontaining impurities which are less volatile than the pure gas material,

means of heat exchange, expansion, andthe like,

that a resulting mixture of two or more phases is obtained; such treatment being effected by an agent, such as heat bearing compressed gas taken from another portion of the manufacturing cycle that is circulated in the jacket 13 to supply heat. When the gas material in chamber 11 obtains sufllciently in the gas phase, it passes over to-' gether with the less volatile impurities into the separator 17; the impurities tending to concentrate in the liquid and solid phases. Here the liquid phase falls by gravity 'to the bottom of the separator, as shown at 20, while the gas phase passing up through the baflles l9 finds exit through the conduit 21. The liquid and solid phase at 20 accumulates the impurities which are less volatile than the liquid phase of the gas material, so that a withdrawal conduit 22 is provided for the separator at its lower end whereby this liquid may be withdrawn when desired.

For the purposes of the invention it would be equally practical to supply refrigeration in chamber 11 by a direct admission of the cooling medium through line 14 into chamber 11. Thus a certain portion of the gas material in liquid form might be expanded directly into chamber 11, thus providing the necessary cooling effect for partial condensation, without the use of indirect heat exchange equipment. e

The practice of the process of the present invention by means of the apparatus illustrated in Fig. 1 will be more readily understood by reference to the phase diagram in Fig. 2, which is drawn with temperatures as abscissae and pressures as ordinates. Here, the line 0C denotes the solid-gas equilibrium curve for the gas material to which the process is applied, C denoting the triple point. CA, likewise, denotes theliquicl-gas equilibrium curve, while CS denotes the liquid-solid equilibrium curve. Where the gas material contains a less volatile impurity, for example, an odorous impurity, its liquid-gas equilibrium curve would, in general, fall below the liquid-gas curve for the gas material itself when drawn on the same chart. The broken line BA is drawn to represent such a curve for the impurity.

'may condenseat a given pressure P1.

the difference in liquid-gas equilibrium curves If, therefore, gas material containing a small but nevertheless objectionable quantity of less volatile impuritiesis admitted to chamber 11 at a pressure P1, such material would be in the gas phase if above the temperature T1, and would be in the liquid phase if at a temperature lower than T1. Whatever the initial condition of the gas material entering chamber 11, however, a transfer of heat may be effected such that a portion of the gasmaterial may be vaporized, whereby the resulting mixture may contain both liquid and gaseous phases. Whereas, this gas material may contain certain less volatile impurities which have a liquid gas equilibrium curve such as BA considerably below the gas material itself, it is obvious that such impurities may separate out into either the liquid or solid phase at a temperature considerably higher than the temperature T1 at which the gas material itself Due to between the gas material and its less volatile impurities, therefore, heat transfer may be effected in chamber 11 such that a mixture of two or more phases may be obtained in substantial equilibrium, such that the less volatile impurities tend to separate out almost entirely in the liquid or solid'phase, leaving the gaseous phase in a state of almost complete freedom from these impurities. Such a separation might even be accomplished at a temperature considerably above the condensation temperature T1 of the gas material, or might be accomplished at the temperature T1 in which case a portion of the gas material itself might be actually condensed into the liquid phase. The optimum operating conditions must depend upon the nature of the gas material and the impurities, and the relative ease with which the impurities may be separated into the liquid or solid phases.

The gas and liquid or solid phases are therefore readily separated in separator 17, and hence any material withdrawn through the conduit 21 is the gas-material in a gas phase in the purified condition desired. The material withdrawn through the conduit 22, therefore, contains substantially all the undesired impurities, this material being withdrawn at will from the separator. Since it may be desired to avoid undue accumulation of material in the separator, the latter-is preferably provided with visual means such as a gauge glass whereby the total liquid content of the separator may be ascertained whenever desired. I

Where the undesired impurities represent an aggregate of those both more and less volatile than the gas material itself, an arrangement of apparatus fOrpracticing the purification steps of the present invention in two stages is shown in Fig. 3. Here, the gas material containing the undesired impurities is supplied througl'i'i'conduit 25 having a controlling valve 26, for example, an expansion valve, and passed into a rectifying column shown generally at 2'7. This column has a vaporizer or kettle 28 at its lower end, and is supplied with a condenser 29 at its upper end. This column is arranged to rectify the liquid whereby the gas fraction constituting the undesired more volatile impurities is first is seen also to set free a gas fraction, which,

as in the form shown in Fig. 1, is the purified gas fraction desired.

Here, the

to flow back to the vaporizer 28, while the more volatile impurities are withdrawn in the gas phase through the conduit 32 from the top the condenser.

The second stage of purification accomplishes the removal of the less volatile impurities by inducing rectification and separation in column 30. This column is supplied with liquid from the vaporizer 28, which contains the less volatile impurities that are withdrawn through the conduit 33' and. enters the rectifying column 30 at the top and is provided with an expansion valve 34 and a distributor 35. The rectification accomplished in the column 30 is so arranged with respect to that taking place in the column 27 that the liquid phase descending in columnBG is brought into thermal contact with the ascending vapors in the column 27, whereby a heat transfer place between the ascending vapors in column. 2? and the gas material in column 30. heat transfer may be effected by maintaining the pres sores and tempe itures in column 30 lower than those in column 25?. Hence, the prevailing tern perature in the bottom of the column 30 is colder than theexisting temperature of the ascei'iding vapors in the top or" the column 27, with which the liquid is brought into thermal contact. The heat transfer, which in consequence takes place, results in vaporizing the liquid in the bottom of the column 30 about the condenser. Rectification then follows and a concentration of liquid containing the undesired less volatile impurities gathers in the column 30 about the condenser 29, while a gas fraction uncontaminated by a less volatile impurity is withdrawn through a conduit 36 leading from the top of the column 30. The liquid phase in the bottom of the column 30 is withdrawn at will through a conduit 37.

The manner in which this two. stage practice of the present invention operates will also be readily unders by reference to the phase diagram in Fl '2. Here, the point E on the liquid-gas equilibrium curve A'B' represents a certain condition of the more volatile undesired impurities in the gas material, which enter the column 2'7. This point E, corresponding to a pressure P2 and a temperature T2 may obtain in the column27 when this material is introduced and collected in the evaporator 28, and is here taken as exemplary. By supplying heat to the liquid through the instrumentality of the coil 31, the volatile constituent is vaporized, together with i a small portion of the gas material which passes upwardly in therectification column, the latter ,being readily condensed in the condenser 29, so as to return to the vaporizer 28. The uncondensed volatile fraction produced being withdrawn through the conduit "32.

The pressure F1 on the diagram may be taken as indicating the'pressure in'the column 30, so that there is a pressure differential correspond ing to i g-P1 causing a flow of the liquid from the evaporator 28 into the top of the column 30. This liquid passing through the expansion valve 34 also undergoes a temperature drop so that the gas material when it reaches the bottom of the column 30 is in the condition indicated by point D on the diagram. Here, itis seen that the heat supplied by the condenser 29 in refrigerating the material in column 27 produces gas material in two phases in the column 30 without vaporizing the less volatile constituent comprising the undesired impurity. A pure gas'fraction, in consequence, is withdrawn through the conduit 36,

The present invention is particularly advantageous in connection with the commercial production of the so-called permanen gases, such as oxygen and nitrogenin substantially pure condition, and is also advantageous in connection with the commercial production of carbon dioxide, from which odorous impurities have heretofore been removed only with dimculty and comparatively great expense. The odorous impurities which are particularly objectionable from a commercial standpoint have been ascertained to belong substantially to the class of the less volatile impurities indicated above, and are hence readily removed by the process of the present invention. I

The phrase gas material is used herein to denote generically a substance thatunder stand ard conditions (i. e., under a pressure of one atmosphere and at a temperature oi 6 C or 275 Kelvin) is normally in the gas phase, but under the conditions described may be in another Thus the phrase gas material describes such substances as air, oxygen, nitrogen, carbon dioxide, etc., whether in the solid, liquid or gas phases.

Since certain changes may be made in carrying out the above process without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

Having described my invention, what I claim as -new and desire to secure by Letters Patent,'is:

, 1, The process of removing impurities occurring in small amounts from gas material collected from a commercial source, which process com prises collecting in a substantial body the gas material including the impurity, said impurity occurring in said material at a concentration such that the boiling point of said gas material is not substantially changed thereby and having a melting point materially above the boiling point of said gas material, bringing said collected gas material substantially in the liquid phase into a heat exchanger in a condition in which the vapor pressure of said impurity has a substantially negligible value, heating the gas material in said exchanger in a manner such as to cause both liquid and gas phases to coexist therein without altering substantially the vapor pressure of said impurity, the said impurity then being contained wholly within the liquid phase of said material while the vapor pressure of said impurity is maintained at a substantially negligible value, and thereatfer withdrawing the vapors associated with said liquid phase and separating the same from said liquid phase whereby gas material free from said impurity is obtained.

2. The process of removing impurities occurring in small amounts from gas material collected from commercial sources which comprises collecting gas material which contains impurities both more volatile and less volatile than the gas material when liquefied, the impurities occurring in said gas material at concentrations such that the boiling point of said gas material is not substantially changed thereby, the less volatile impurity having a melting point materially above the boiling point of said gas material, bringing said collected material substantially in the liquid phase into a heat exchanger to a temperature and pressure corresponding substantially to a point on the liquid-gas phase equilibrium curve for said material, withdrawing the more volatile impurity while in .a condition in which the vapor pressure of the less volatile impurity has r d substantially negligible, value, then heating'the said material in a second stage so that both liquid and gas phases co-exist, the less volatile impurity being contained wholly within the liquid phase of said material while the vapor pressure 01' said less volatile impurity remains substantially unaltered, and thereafter separating the gas phase associated with said liquid phase in said second stage liquid oxygen is not substantially ced thereby and having a melting point materially above the boiling point 0! liquid oxygen, bringing said liquid oxygen containing said impurity in a heat exchanger to a temperature and pressure corresponding substantially to a point on the liquid-gas phase equilibrium curve while in a condition in which the vapor pressure of said impurity has a substantially negligible value, adding heat to the introduced oxygen in a manner such that both liquid and gas phases coexist while said impurity is contained wholly within the liquid phase of the oxygen and the vapor pressure of, said impurity remains substantially unaltered, and thereafter separating oxygen in the gas phase irom the liquid phase whereby gaseous oxygen free from the undesired impurity is obtained.

LACE J. BOITCH, 

